Power switching circuits such as bridge circuits are commonly used in a variety of applications. A circuit schematic of a 3-phase bridge circuit 10 configured to drive a motor is shown in FIG. 1. Each of the three half bridges 15, 25, and 35 in circuit 10 includes two transistors, 41 and 42, 43 and 44, and 45 and 46, respectively, which are able to block voltage in a first direction and are capable of conducting current in the first direction or optionally in both directions. In applications where the transistors employed in the bridge circuit 10 are only capable of conducting current in one direction, for example when silicon IGBTs are used, an anti-parallel diode (not shown) may be connected to each of the transistors 41-46. Each of transistors 41-46 is capable of blocking a voltage at least as large as the high voltage (HV) source 11 of the circuit 10 when they are biased in the OFF state. That is, when the gate-source voltage VGS of any of transistors 41-46 is less than the transistor threshold voltage Vth, no substantial current flows through the transistor when the drain-source voltage VDS (i.e., the voltage at the drain relative to the source) is between 0V and HV. When biased in the ON state (i.e., with VGS greater than the transistor threshold voltage), the transistors 41-46 are each capable of conducting sufficiently high current for the application in which they are used.
The transistors 41-46 may be enhancement mode or E-mode transistors (normally off, Vth>0), or depletion mode or D-mode (normally on, Vth<0) transistors. In power circuits, enhancement mode devices are typically used to prevent accidental turn on which may cause damage to the devices or other circuit components. Nodes 17, 18, and 19 are all coupled to one another via inductive loads, i.e., inductive components such as motor coils (not shown in FIG. 1).
FIG. 2a shows half bridge 15 of the full 3-phase motor drive in FIG. 1, along with the winding of the motor (represented by inductive component 21) between nodes 17 and 18. Also shown is transistor 44, into which the motor current feeds. For this phase of power, transistor 44 is continuously ON (Vgs44>Vth) and transistor 42 is continuously OFF (Vgs42<Vth, i.e., Vgs42=0V if enhancement mode transistors are used), while transistor 41 is modulated with a pulse width modulation (PWM) signal to achieve the desired motor current. FIG. 2b indicates the path of the current 27 during the time that transistor 41 is biased ON. For this bias, the motor current flows through transistors 41 and 44, while no current flows through transistor 42 because transistor 42 is biased OFF, and the voltage at node 17 is close to HV, so transistor 42 blocks a voltage which is close to HV.
As used herein, the term “blocking a voltage” refers to a transistor, device, or component being in a state for which significant current, such as current that is greater than 0.001 times the average operating current during regular ON-state conduction, is prevented from flowing through the transistor, device, or component when a voltage is applied across the transistor, device, or component. In other words, while a transistor, device, or component is blocking a voltage that is applied across it, the total current passing through the transistor, device, or component will not be greater than 0.001 times the average operating current during regular ON-state conduction.
Referring to FIG. 2c, when transistor 41 is switched OFF, no current can flow through transistor 41, so the motor current flows in the reverse direction through transistor 42, which can occur whether transistor 42 is biased ON or OFF. Alternatively, an anti-parallel freewheeling diode (not shown) can be connected across transistor 42, in which case the reverse current flows through the freewheeling diode. During such operation, the inductive component 21 forces the voltage at node 17 to a sufficiently negative value to cause reverse conduction through transistor 42, and transistor 41 blocks a voltage which is close to HV.
FIGS. 3a-3c show operation of the half bridge 15 under conditions where current passes through the inductive load in the opposite direction as compared to that shown in FIGS. 2a-2c, and the voltage at node 17 is controlled by switching the low-side transistor 42. For the mode of operation illustrated in FIGS. 3a-3c, the motor current 27 is fed into the inductive motor 21 through transistor 43. During this mode of operation, transistor 43 is continuously ON (Vgs43>Vth) and transistor 41 is continuously OFF (Vgs41<Vth, i.e., Vgs41=0V if enhancement mode transistors are used), while transistor 42 is modulated with a pulse width modulation (PWM) signal to achieve the desired motor current. FIG. 3b indicates the path of the current 27 during the time that transistor 42 is biased ON. For this bias, the motor current flows through transistors 43 and 42, while no current flows through transistor 41 because transistor 41 is biased OFF, and the voltage at node 17 is close to 0 V, so transistor 41 blocks a voltage which is close to HV.
Referring to FIG. 3c, when transistor 42 is switched OFF, no current can flow through transistor 42, so the motor current flows in the reverse direction through transistor 41, which can occur whether transistor 41 is biased ON or OFF. Alternatively, an anti-parallel freewheeling diode (not shown) can be connected across transistor 41, in which case the reverse current flows through the freewheeling diode. During such operation, the inductive component 21 forces the voltage at node 17 to a sufficiently high value (slightly higher than HV) to cause reverse conduction through transistor 41, and transistor 42 blocks a voltage which is close to or slightly higher than HV.
In addition to their use in motor-drive applications, half bridges and bridge circuits can also be used in many other applications, for example boost or buck converters or in power supplies. An exemplary circuit which utilizes a half bridge 15 to drive an electrical load 28 is illustrated in FIG. 4. The electrical load 28 can, for example, be capacitive and/or resistive, or in some cases could be a battery or DC power supply. As further illustrated in FIG. 4, in many applications a filter 22, which can include inductive and/or capacitive elements 23 and 24, respectively, is inserted between the half bridge 15 and the electrical load 28.
The mode of switching illustrated in FIGS. 2a-2c and 3a-3c is commonly known as hard-switching. A hard-switching circuit configuration is one in which the switching transistors are configured to have high currents passing through them as soon as they are switched ON, and to have high voltages across them as soon as they are switched OFF. In other words, the transistors are switched ON during periods where non-zero currents flow through the inductive load, so substantial current flows through the transistors immediately or soon after the transistors are switched ON, rather than the current rising gradually. Similarly, the transistors are switched OFF during periods where high voltages must be blocked by the transistors, so substantial voltage is blocked by the transistors immediately or soon after the transistors are switched OFF, rather than the voltage rising gradually. Transistors switched under these conditions are said to be “hard-switched”.
Alternative circuit configurations make use of additional passive and/or active components, or alternatively signal timing techniques, to allow the transistors to be “soft-switched”. A soft-switching circuit configuration is one in which the switching transistors are configured to be switched ON during zero-current (or near zero-current) conditions and switched OFF during zero-voltage (or near zero-voltage) conditions. Soft-switching methods and configurations have been developed to address the high levels of electro-magnetic interference (EMI) and associated ringing observed in hard-switched circuits, especially in high current and/or high voltage applications. While soft-switching can in many cases alleviate these problems, the circuitry required for soft switching typically includes many additional components, resulting in increased overall cost and complexity. Soft-switching also typically requires that the circuits be configured to switch only at specific times when the zero-current or zero-voltage conditions are met, hence limiting the control signals that can be applied and in many cases reducing circuit performance. Hence, alternative configurations and methods are desirable for hard-switched power switching circuits in order to maintain sufficiently low levels of EMI.